After Brienne defeats Loras Tyrell in single combat, King Renly is impressed by the warrior-woman's prowess and states that she is everything her father said she was and more.[1] When Jaime Lannister meets Brienne for the first time, he spends a moment trying to recall Tarth, then remembers that its lord is her father, Selwyn of House Tarth.[2]

When the captured Brienne is about to be raped by Locke and then by his men, Jaime successfully stops them by promising Lord Selwyn Tarth would pay a high ransom for his only surviving child's return, but only if Brienne's honor remains "unbesmirched." Jaime tells Locke that Tarth is famous as "the Sapphire Isle" because it is the location of all sapphire mines in Westeros, and Lord Sewlyn will give him Brienne's weight in sapphires for her release. Locke agrees and calls off his men before they can rape Brienne.[3]

Brienne subsequently points out to Jaime in private that Tarth is called "the Sapphire Isle" because of the blue of the surrounding waters - there are no sapphire gemstones. She realizes Jaime must have known this, but embellished her father's wealth in order to convince Locke to leave her unharmed.[4]

Lord Selwyn responds to Locke's ransom demand via raven, offering a ransom of three hundred Gold Dragons in exchange for Brienne's release. Unfortunately, Locke became so convinced by Jaime's previous spur of the moment lie to save Brienne that he refuses Selwyn's offer, and will settle for nothing less than literally Brienne's weight in sapphire gemstones. Selwyn's messages insist there are no sapphires on Tarth but Locke simply thinks he is refusing to pay. Locke and his men decide to entertain themselves by putting Brienne into Harrenhal's bear pit, but she is saved by the Jaime's intervention.[5]

Brienne mentions her father when she encounters Arya Stark. She reminisces how Lord Selwyn told her that it was not proper behavior for a lady to pursue martial activity. However, Brienne did not listen and continued fighting other boys until her father relented and had her properly trained as a warrior.[6]

In the A Song of Ice and Fire novels, Selwyn Tarth is described as a good man of fifty-four years old. Also known as "The Evenstar", he is a widower remembered by Brienne as having a new woman every year. He initially declares House Tarth's support for Renly. After Renly dies, it isn't clearly stated whom he supports. He certainly didn't send troops to aid Stannis, but it's unclear if he bent the knee to Joffrey, outright lent his support to him, or simply adopted an isolationist stance, like Lord Gulian Swann.